1. Field of the Invention
This invention is in the field of power production from geothermal energy sources.
2. Description of the Prior Art
In a given geothermal field where a number of geothermal hot water wells are drilled, substantial temperature differences can be expected to be found in the hot water produced by different wells. Such temperature variations may be caused by a variety of factors, such as different rates of heat flow to the surface of the ground, different types of rock strata, different depths at which the hot water-bearing strata occur, and different distances of the wells from the high temperature geographical center of the field. Thus, for example, in the Imperial Valley of Southern California there are geothermal hot water fields where large amounts of geothermal hot water at temperatures on the order of about 370.degree. F. are found at depths of about 5,000 to 6,000 feet; while in the same fields at depths of only about 2,000 feet even larger amounts of geothermal hot water are found at temperatures on the order of about 300.degree. F. Also, a geothermal hot water field will typically have a large peripheral area where the geothermal hot water resource is considerably cooler than that of the center part of the field, but still at sufficiently high temperatures to be useful in the production of power. Utilization of these upper level and peripheral area cooler geothermal resources can increase the power output from a given geothermal field on the order of two to three times.
Another factor which may result in geothermal hot water wells of a given field having different temperatures is where geothermal hot water may be withdrawn from some wells at a greater rate than the heat flows normally from the water to the surface. With such a condition it can be expected that the supply temperature at the bottom of the well may decrease with time, and this could be expected to differ for various wells in a given field, so that it would result, in time, in different temperature wells in the field.
It is known in the art to produce power from geothermal hot water wells by transferring heat from the geothermal water in surface heat exchangers to a power or working fluid so as to boil and superheat the power fluid under high pressure, the power fluid being employed in a closed Rankine cycle to develop power. In such a system the geothermal hot water may be pumped from the well as taught in Barkman C. McCabe Pat. No. 3,757,516 to prevent flashing in the well or in the heat exchangers; or it may be allowed to flow up out of the well under the power of its own flashing steam, and then either the combined hot water and steam may be flowed through the heat exchangers or it may be passed through steam separators, with the steam then being passed through the heat exchangers.
The James Hilbert Anderson U.S. Pat. No. 3,795,103 discloses how the efficiency of such a system where the heat energy of geothermal hot water is transferred to a separate power fluid cycle can be improved by employing two or more power fluids which boil at different temperatures in respective separate closed Rankine cycles, and diverting a portion of the geothermal hot water from a higher temperature power fluid cycle to a lower temperature power fluid cycle.
The conventional approach where the heat energy from a plurality of geothermal wells is to be transferred through heat exchangers into a power fluid cycle would be to mix the geothermal fluid, whether it be pumped hot water, a combination of hot water and steam, or separated steam, from the wells prior to entry into the cycle process, i.e., prior to heat exchange with the power fluid. The temperature of the geothermal fluid mixture at the point of entry into the cycle process would thus be an average temperature between the temperatures of the high temperature well and the low temperature well, and in a typical situation where the flow rate for the low temperature well would be greater than that for the high temperature well, this average geothermal fluid temperature would be closer to the temperature of the low temperature well than to that of the high temperature well.
This substantially lower temperature for the fluid mixture from the plurality of wells than the temperature of the fluid from the hottest well would result in a correspondingly lower temperature to which the power fluid could be superheated in the cycle, with a resulting loss in the amount of power that could be derived from the power fluid cycle. In a multiple power fluid cycle like that disclosed in James Hilbert Anderson Pat. No. 3,795,103, this reduced entry temperature of the geothermal fluid mixture into the cycle would generally require that a disproportionate volume of the geothermal fluid mixture be directed to a higher temperature power fluid part of the system at the expense of a lower temperature power fluid part of the system, with a resulting reduced efficiency in the overall system.
The efficiency would be similarly reduced in a waste heat power plant where a plurality of hot fluid streams at different temperatures are available for generating power at relatively low temperatures, if the streams were mixed before their heat energy was transferred into a power fluid cycle.